1. This invention relates to systems with a multiplicity of devices where there is a need to determine the states of the devices during regular operation or during partial or complete rest periods of the system. A particular application of this invention is the determination of the state of charge of batteries either incorporated in operating systems or as isolated units.
2. Modern systems with their high degree of complexity and sophistication have created the need for testing the condition or state of very many critical components to insure the proper, safe, operation and maintenance of the systems. Testing or Self Testing is, for example, applied in the wide area of digital electronics where automatic testing of systems with the aid of computers is widely used and also currently undergoing extensive developments. Another, more particular, example of the importance of testing is the case of systems incorporating transistors. These devices produce internal heat in addition to heat supplied by the hot environments in which they are usually operating. The internal temperatures of the transistors are therefore elevated and may reach levels beyond which permanent damage to a transistor would occur. It often becomes important, therefore, to measure the internal temperature of a transistor, known as the Junction Temperature in bipolar transistors. This is usually accomplished by measuring the transistor case temperature and by adding to it temperature increments calculated from known or measured internal power dissipations. This method is in many cases inaccurate and results in transistor failures that could be prevented if more precise knowledge of junction temperatures were available. Another example of a component critical to the operation of a system is a battery. The state of charge of system batteries should be known at all times so that a replacement or recharge operation could be undertaken in time to ensure against system failures. Knowledge of the state of charge should be accurate because this would allow the batteries to stay in use, reliably, longer and very close to a complete discharge, for example. This should increase the overall economy and efficiency of system operations and is a particularly important factor in the case of complex and expensive remote systems.
Several methods for measuring the state of charge of batteries have been described but all suffer either from lack of accuracy or the lack of flexibility in terms of integration into large systems. In many cases battery terminal voltage has been used to determine the state of charge. This method can, in principle, be integrated into a system by recording the electrical voltages of the system batteries but the resulting accuracy is mostly insufficient. Another method is described in U.S. Pat. No. 3,562,634 by Norman Latner and entitled: Method for Determining the State Of Charge of Nickel Cadmium Batteries By Measuring the Farad Capacitance Thereof. In that disclosure the capacitance between a nickel-cadmium battery electrode and electrolyte was measured electrically and used to determine the state of charge. Specific measurement conditions were disclosed that indicate the feasibility of that method but no instrumentation was disclosed that would enable the integration of the measurement method into operating systems. Battery temperature affects the measurement of electrode capacitance and this has not been discussed by Latner. Temperature effects come about mainly through the resistive components of the battery impedance through which the approximate capacitance can be determined. This is true in general for any battery including nickel-cadmium batteries. Another complicating factor is the fact that the equivalent circuit of battery impedance is not a simple lumped capacitance with resistive leads and leakage but rather a network of distributed capacitors, resistors and inductors. A simple or even a high quality impedance bridge measurement as disclosed by Latner can, therefore, not be made to produce a pure capacitance value corresponding to the state of charge. The particular problem involved with the determination of battery temperature is the fact that the internal, effective, electrode and electrolyte temperature has to be measured. This can be done under special, laboratory, conditions where the battery is in thermal equilibrium i.e. without internal heat dissipation. This is never the case in practice where heat flow and internal dissipation are always present. The effective temperature, therefore, not be measured directly. Another example of this problem is the case of a transistor where internal heat dissipation raises the junction temperature by an amount that cannot be measured directly. In this case terminal impedance can be represented by the transistor transconductance or by the base to collector current gain factor, for example. This, generalized, impedance is a function of the junction temperature and additional electrical parameters such as the base and collector currents. The above impedance is also a function of frequency and becomes a particularly complex function of all the above parameters at certain high frequencies. Generalized impedance, then, contains information on many of a device's parameters. Measurement of impedances lends itself also to integration in large systems. However, the extraction of information, from the measurement, on individual parameters is generally a complex problem that has not found, in the prior art, an adequate solution.